It is standard practice to put a membrane down before adding stones in landscaping. Not only does this lessen the weeds (not prevent) but it also stops the stones and soil below mixing.
Decide what you want for this area and tell Barratt to do it.
If they refuse, say you will give them a big fat "NO" to Q1 in the 8-week satisfaction survey which they are so keen on!
The site manager's bonus is dependent on "Yes" answers, so it will actually cost him money to save Barratt a few pounds!